This invention relates to antennas and in particular to an arrangement to electrically down-tilt the electromagnetic wave pattern associated with a transmit antenna array, or electrically re-orient a receive antenna array.
It is sometimes desirable to adjust the orientation of the electromagnetic wave pattern of a transmit antenna array, particularly a downward adjustment, typically 0 degrees to 15 degrees below horizontal, when the antenna is located at a higher altitude than other antennas that communicate with the transmit antenna array. The downward adjustment of the radiation pattern alters the coverage area and may enhance communication with mobile users situated in shadowed areas below the transmit antenna array.
Besides actually mechanically tilting the entire antenna assembly, it is known to electrically down-tilt the radiation pattern by controllably varying the relative phase or phases between two or more radiating elements of the antenna array.
One known method by which the relative phase between two or more radiating elements can be change to change the relative lengths of respective transmission lines connecting the antenna""s common feed point to each element of the antenna array. Typically, various predetermined lengths of jumper cable are provided which are selectively connected between the common feed and each element to obtain a desired down-tilt. The jumper cables include co-axial connectors to facilitate connection. Furthermore, if stripline is used to connect the common feed point to the respective elements of the antenna array, some form of transition means is required to couple the jumper cable""s coaxial connections to the stripline. A disadvantage of this known method is that it is expensive, unreliable and susceptible to the generation of intermodulation products.
Another known method by which the relative phase between two or more radiating elements can be changed is to change the propagation velocity of the transmission line connecting the common feed point to at least some of the elements of the antenna array. Typically, this latter method is achieved by selectively changing the dielectric constant of the transmission line dielectric. If the transmission line is in the form of a conductive strip, the propagation velocity thereof is changed by introducing a dielectric material between the strip and its associated ground plane.
It is, however, well understood that the introduction of dielectric material under such a conductive strip causes the strip""s normal impedance to be disturbed. For example, if a conductive strip having a certain width is spaced above a ground-plane at a certain distance such as to present a 50 ohm impedance, the introduction of a dielectric material between the conductive strip and the ground-plane will reduce the value of this impedance to a value that depends upon the effective dielectric constant of the dielectric material. The resulting impedance mismatch would cause a degradation of return-loss performance of the antenna.
Australian Patent No. 664625 discloses an arrangement of an adjustable phase shifter comprising dielectric phase shifter elements moveably interposed between conductive strips that couple radiating elements, and a common ground-plane. The phase shifter elements are of a characteristic configuration which avoids disturbing the normal impedance during adjustment. This known arrangement, however, requires that respective phase shifter elements be located between each active strip and the conductive ground-plane. Such an arrangement imposes constructional disadvantages as well as limitations to the range of phase shift produced, which consequently imposes limits to the range of tilt.
Australian Patent Application No. 14278/99 discloses an arrangement of an adjustable phase shifter comprising a transmission line in the form of a printed circuit board supporting conductive tracks on one side thereof, and a ground-plane spaced below the other side thereof. A moveable dielectric element is arranged adjacent the conductive tracks. The moveable dielectric element is provided with a plurality of teeth along opposite edges for selectively overlapping the conductive tracks. This arrangement provides an adjustable phase shifter having stable impedance characteristics and a relatively large phase shift as compared with the prior art.
New radio frequency bands have been allocated to provide more channels for the rapidly increasing cellular mobile telephone usage. Instead of having separate base station antennas for different bands, it is desirable to provide multiple-band antennas. For example, it may be desirable to combine the 1710-1880 MHz DCS band with the 1920-2170 MHz UMTS band, with an overall bandwidth of 460 MHz, which is wider than previous systems. In order to electrically down-tilt the radiation pattern of such a multiple band antenna, a phase shifter is required that has a wider frequency range than the aforementioned prior art phase shifter arrangements can accomplish.
It is an object of the present invention to provide an adjustable radio frequency phase shifter arrangement having a wider operating frequency range than prior art arrangements.
According to the invention there is provided a phase shifter element arranged to selectively vary the effective dielectric constant of a section of transmission line thereby changing the propagation velocity of the transmission line and varying the phase of signals of desired frequencies or frequency range passing through the transmission line the phase shifter element comprising a movable planar dielectric member of predetermined dielectric constant adjacent the transmission line, the planar dielectric member being provided with three or more discrete interactive dielectric segments extending from at least one edge thereof to moveably overlap the adjacent transmission line, where optimum dimensions of each interactive segment and optimum widths of gaps defined by opposite edges of adjacent segments are determined by computer optimisation means, such that the aggregate reflection of the signals passing through the transmission line is minimised.
The present invention is based on the concept that, as phase shift is generally proportional to the length of added or inserted dielectric, if the total dielectric length required for one particular phase shift is broken up into a plurality of segments, then it is possible to optimise all of the lengths and spacings of those segments in such a way as to reduce the aggregate reflection at several frequencies, or over a range of frequencies. Thus for a specified frequency range and for one required electrical phase shift, computer optimisation is used to determine the optimum lengths of a plurality of dielectric segments and the spacings between them. The same procedure is repeated for other phase shift values to produce an optimum profile for the movable dielectric element.
In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to figures of the accompanying drawings, in which: